The present invention relates to an optical track sensing device, and in particular to an optical sensor for detecting optical tracking information on a magnetic floppy disk.
A high density floppy disk drive has been developed to store 120 megabytes of data on a 3.5" floppy disk. This high capacity floppy disk uses a special floppy disk with optical tracks recorded on one of its surfaces to allow the magnetic recording head to store information with closer track spacing. An optical track is recorded between every two magnetic tracks to provide more precise positioning information than can be achieved using the magnetic data alone. This allows the magnetic tracks to be placed closer together. In order to place the magnetic head precisely with respect to the optical tracks, an optical sensing device is used to detect the error between the magnetic head and the optical track. A prior art sensing device was described in a paper by S. W. Farnsworth, S. D. Wilson and B. Cohen entitled "Diffractive Optical System for Tracking on Floptical Disk", Optical Instrument, SPIE Vol. 1690, pp.72-79, 1992.
FIG. 1 shows this prior art track sensing device. A semiconductor laser source 11 is imaged by a lens 13 and a rooftop mirror 14 to the floppy disk 15. A diffractive optical element 12 is used to divide the laser beam into many beams. While some beams are reflected upwards towards the floppy disk, others are reflected downwards ty the lower half of the rooftop mirror to an encoder 16. The encoder provides tracking information to the floppy disk drive when a low density floppy disk without pre-recorded tracks is inserted into the disk drive. The encoder is a strip of optical tracks along the inside housing of the floppy disk crive which allows the position of the magnetic head to be determined optically, even though no optical tracks are on the low density floppy disk itself. The optical sensor is mounted on the magnetic head itself, and a hole is provided through the arm supporting the magnetic head for the optical path. This special optical track sensor permits this high density floppy disk drive to be backward compatible with the low density floppy disks. The returned beams from the floppy disk and the encoder are again reflected by the rooftop mirror 14 and are projected by a lens 17 to a multiple element detector 18.
The detector typically has six elements arranged in two rows. A first row of three elements is used to detect the tracking information from the floppy disk, while the second row is used to detect the encoder on the floppy disk drive housing. A double-slit optical element is used to provide an interference pattern, with the spacing of the interference lobes matching the spacing of the optical tracks. In this way, multiple tracks can be detected at once. Thus, each of the six elements of the detector should see five separate images corresponding to five tracks. The intensity of the received signal will be low, indicating a tracking error, if one or more beams is missing. Thus, the optical sensor splits the laser beam in two, for the floppy disk and the encoder on the housing. Next, each of the two beams are split into three to provide beams which can be detected by the three optical elements in each row of the detector. Finally, each of the six beams is split into five lobes to enable detection of each of five optical tracks.
One of the characteristics of this prior art optical track sensor is that the optical axis of lenss 12 and the optical axis of the lens 17 subtend a small angle with respect to the mechanical axis. Consequently, the laser beams produced by the diffractive optical element 12 are not perpendicular to the surface of the floppy disk or the encoder. As a result, the lateral position of the beams on the floppy disk will change when the floppy disk moves up and down with respect to the image plane of the lens 13. Because of the multiple beams and lobes used, even a small variation in up and down movement of a floppy disk can affect the accuracy of the optical tracking.